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 A032667 Digit '4' concatenated with a(n) is a prime. 2
 1, 3, 7, 19, 21, 31, 33, 39, 43, 49, 57, 61, 63, 67, 79, 87, 91, 99, 111, 127, 129, 133, 139, 153, 157, 159, 177, 201, 211, 217, 219, 229, 231, 241, 243, 253, 259, 261, 271, 273, 283, 289, 297, 327, 337, 339, 349, 357, 363, 373, 391, 397, 409 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Obviously there can be no even terms in this sequence. - Alonso del Arte, Jun 18 2017 LINKS Seiichi Manyama, Table of n, a(n) for n = 1..10000 (terms 1..1740 from Vincenzo Librandi) EXAMPLE Concatenate 4 and 1 to get 41, which is prime, so 1 is in the sequence. Concatenate 4 and 3 to get 43, which is prime, so 3 is in the sequence. Concatenate 4 and 5 to get 45 = 3^2 * 5, which is not prime, so 5 is not in the sequence. MATHEMATICA Select[2Range[250] - 1, PrimeQ[FromDigits[Join[{4}, IntegerDigits[#]]]] &] (* Alonso del Arte, Jun 18 2017 *) PROG (PARI) isok(n) = isprime(eval(concat(4, Str(n)))); \\ Michel Marcus, Jun 19 2017 CROSSREFS Cf. other digit 'd' concatenated with a(n) is prime sequences: A032664 (d = 1), A032665 (d = 2), A032666 (d = 3), A032668 (d = 5), A032669 (d = 6), A032670 (d = 7), A032671 (d = 8), A032672 (d = 9), A000040 (d = 0). Sequence in context: A242170 A032675 A089749 * A109991 A248219 A322963 Adjacent sequences:  A032664 A032665 A032666 * A032668 A032669 A032670 KEYWORD nonn,base AUTHOR Patrick De Geest, May 15 1998 EXTENSIONS Offset adjusted at the suggestion of Michel Marcus by Alonso del Arte, Jun 18 2017 STATUS approved

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Last modified July 22 20:51 EDT 2019. Contains 325226 sequences. (Running on oeis4.)